OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Offshore Platform Fluid Structure Interaction (FSI) Simulation Ali Marzaban, CD-adapco Murthy Lakshmiraju, CD-adapco Nigel Richardson, CD-adapco Mike Henneke, CD-adapco Guangyu Wu, Chevron Pedro M. Vargas, Chevron Owen Oakley, Chevron August 13, 2013 1
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Introduction • The main objective of this study was to predict the permanent deformation of an offshore platform from a large wave incident during a storm using Fluid Structure Interaction (FSI). • A representative sub-modeled 1/8 th section of the offshore oil platform that was deformed permanently during a hurricane is demonstrated. • Study was divided into three phases to progressively develop a FSI methodology to predict the permanent deformation of the platform : – Phase 1: Static and transient dynamic structural investigation was conducted on a sub-model section of the platform to predict potential wave energy required to cause the field observed deformation (around 40 cm deformation was reported on one of the plate girders). • ABAQUS 6.11 was used for Finite Element Analysis (FEA). – Phase 2: Hurricane waves were simulated using Computational Fluid Dynamics (CFD) to determine wave characteristics required to induce the magnitude of pressure needed to observe same deformation on a structure (Based on Phase 1). • STAR-CCM+ 6.06 was used for CFD analysis – Phase 3: One-way coupled simulations were modeled to study impact analysis on the structure due to wave pressure. August 13, 2013 2
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Phase1 – Finite Element Analysis • A 3D finite element model of an offshore platform was generated from Chevron provided 2D drawings. • The model was investigated for a single wave impact event using ABAQUS 6.11 to study the permanent plastic deformation. • The model has been analyzed using both static and transient dynamic (implicit) approach. • Varying pressure loads were applied in the static cases to validate recorded field data deformation (40 cm on one of the plate girders). • Pressures were applied at different periods during the implicit analysis studies. August 13, 2013 3
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Phase2 – Computational Fluid Dynamics • A uni-directional wave field was generated based on 60 individual wave components (provided by Chevron) using a Fortran based user sub-routine code. • 2D simulations were performed without the platform to determine the position of the peak wave occurrence to place the structure. • 2D simulations also helped in estimating the mesh size and time step for coupling fluid-structure interaction phase. • Platform was situated such that maximum energy would be imparted on the structure. August 13, 2013 4
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Phase3 – One-way Coupled Fluid Structure Interaction • One-way coupling scheme: Fluid imparted pressures on the structure will be transferred to the FE model but the response of the structure to the fluid will be neglected. – Structure is treated as a rigid body in the CFD model • One-way coupling analysis was performed to predict the observed deformation on plate girders using STAR-CCM+ and ABAQUS co-simulation. August 13, 2013 5
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Laser Scan Measurement of Plate Girders August 13, 2013 6
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Phase 1 August 13, 2013 7
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Modeling Assumption and Material Properties • The 3D FEA model was generated from 2D drawings provided by Chevron. • C3D20R elements were used to model all solid parts. • Local connection details (bolts and welds) were not considered. All joint were considered to be infinitely stiff. • Elastic-plastic material properties was used for plate girders. For plate girders A588 typical material properties was used: • Initial Elastic Modulus: 200 GPa • Poisson’s ratio: 0.3 • Density : 7830 kg/m 3 • Yield Stress: 379.2 MPa • Ultimate Stress: 530.9 MPa August 13, 2013 8
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Finite Element Mesh – Full Assembly ELEMENTS: 21,114 NODES: 118,200 August 13, 2013 9
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Applied Boundary Conditions Fixed in x-direction Uniform Pressure in x-direction Applied pressure Fixed in all DOF • Gravity is applied in negative z-direction. • Uniform pressure distribution was applied to the plate girder in positive x-direction. August 13, 2013 10
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Displacement for 300kPa Pressure in Static Analysis t=0.2 s t=0.3 s t=0 s t=0.1 s t=0.4 s t=0.5 s t=0.6 s t=0.7 s t=1 s t=0.8 s t=0.9 s August 13, 2013 11
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Plastic Strain in Static Analysis August 13, 2013 12
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Static Analysis • In the static analysis, the load was applied as a uniform pressure. • Five different load cases with different maximum pressure of (100, 150, 200, 250 and 300kPa) was considered. • The pressure was applied as a ramp from zero to max from t=0 to t=0.5s. Then the pressure was released as a ramp from the max pressure to zero from t=0.5s to t=1s. A 43cm deflection Pressure=277kPa Displacement of node A in x-direction with different pressures Deformed location of node A in x-direction with different pressures August 13, 2013 13
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Implicit Analysis • In the implicit analysis, the load was applied as a uniform pressure with the maximum pressure of 300kPa. • Four different load cases with different total times of (T=0.01, 0.05, 0.1 and 1 second) was considered. • The pressure was applied as a ramp from zero to max (300kPa) from t=0 to t=0.5T. Then the pressure was released as a ramp from the max pressure (300kPa) to zero from t=0.5T to t=T. A zero pressure was applied from t=T to t=3T. August 13, 2013 14
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Displacement in x-direction for Different Times in Implicit Analysis 0.5T T 1.5T 2T 2.5T 3T Displacement of node A in x-direction with different Frequency for 300kPa A 0.5T T 1.5T 2T 2.5T 3T Applied Force vs Time August 13, 2013 15
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Required Pressure for 43 cm Displacement Total Time (s) Pressure (kPa) 415 0.05 322 0.1 273 0.5 270 1 5 275 Static 277 August 13, 2013 16
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Phase 2 August 13, 2013 17
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Computational Domain, Mesh and Boundary Conditions – Without Structure Front and Back: Top: Pressure Outlet Symmetry (One cell thick) 500.0 m Inlet: User Code Outlet: Pressure Free surface Outlet 534.62 m X = 1200m X = 800m Bottom: Wall (No-slip)- Ocean bed X = 2000m Mesh : 101,600 Delta X is fixed: 3m and Delta Y is fixed: 2m (One cell thick domain) Mesh size near free surface, Refined area 1 (Z= -25m to 25m): Delta Z = 1m Mesh size near free surface, Refined area 2 (Z=-50m to -25m & 50m to 25m): Delta Z = 1.5m In water, Delta Z increases to 10m (at ocean bottom) In air, Delta Z increases to 20m (at Top) August 13, 2013 18
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation CFD Methodology • Physical Models used: – Three dimensional – Implicit unsteady – Gravity – Multiphase mixture – Eulerian multiphase • Water: Constant density – Density: 997.561 Kg/m 3 and Dynamic viscosity: 8.887E-4 Pa-s • Air: Constant density – Density: 1.18415 Kg/m 3 and Dynamic viscosity: 1.85508E-5 Pa-s – Volume of Fluid – VOF waves - Flat water condition (for generating mean free surface) – Turbulence: – SST (Menter) K-Omega turbulence model with High y+ Wall treatment – Wave Damping • Imposed from 400m from outlet August 13, 2013 19
OMAE 2012 -83472: Offshore Platform Fluid Structure Interaction (FSI) Simulation Wave specifications • Water depth: 1754 ft (534.6m) • Peak wave period: 14.8s • Zero crossing (Mean) wave period: 10.2s • Maximum wave height: 74 ft ±1.5 ft (22.55m ± 0.457m) • Significant wave height: 43.3 ft (13.19m) • Surface current (above – 200ft) velocity: 2.1 knots (1.08m/s) • Static wind speed: 85 knots (43.72 m/s) – Direction of surface current and wind speeds are unknown – Assumption: static current and wave speed direction is same as the wave advancing direction. • Height of the lower deck to free surface: 56 ft (17.07 m) • Wave properties: (provided by Chevron) – Superposition of 60 waves – Highest crest is 16.92m at 1325.6m and 166.0 s – Velocity, height, volume fraction: calculated on a point-by-point basis from given data using a FORTRAN subroutine August 13, 2013 20
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